RFC 3608 SIP Extension for Service Route Discovery October 200311. Intellectual Property Statement. . . . . . . . . . . . . . . 1612. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 1613. Full Copyright Statement . . . . . . . . . . . . . . . . . . 171. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119 [1].
2. Background
The Third Generation Partnership Project (3GPP) established a
requirement for discovering home proxies during SIP registration and
published this requirement in [6]. The 3GPP network dynamically
assigns a home service proxy to each address-of-record (AOR). This
assignment may occur in conjunction with a REGISTER operation, or
out-of-band as needed to support call services when the address-of-
record has no registrations. This home service proxy may provide
both inbound (UA terminated) and outbound (UA originated) services.
In the inbound case, the Request-Uniform Resource Identifier (URI) of
incoming SIP requests matches the address-of-record of a user
associated with the home service proxy. The home service proxy then
(in most cases) forwards the request to the registered contact
address for that AOR. A mechanism for traversing required proxies
between the home service proxy and the registered UA is presented in
[4].
Outbound (UA originated) session cases raise another issue.
Specifically, "How does the UA know which service proxy to use and
how to get there?"
Several mechanisms were proposed in list discussions, including:
1. Configuration data in the UA. This raises questions of UA
configuration management and updating, especially if proxy
assignment is very dynamic, such as in load-balancing scenarios.
2. Use of some other protocol, such as HTTP, to get configuration
data from a configuration server in the home network. While
functional, this solution requires additional protocol engines,
firewall complexity, operations overhead, and significant
additional "over the air" traffic.
3. Use of lookup tables in the home network, as may be done for
inbound requests in some 3G networks. This has a relatively high
overhead in terms of database operations.
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RFC 3608 SIP Extension for Service Route Discovery October 2003
4. Returning a 302 response indicating the service proxy as a new
contact, causing the upstream node processing the 302 (ostensibly
the UA) to retransmit the request toward the service proxy. While
this shares the database operation of the previous alternative, it
does explicitly allow for caching the 302 response thereby
potentially reducing the frequency and number of database
operations.
5. Performing an operation equivalent to record-routing in a REGISTER
transaction between the UA and the associated registrar, then
storing that route in the UA and reusing it as a service route on
future requests originating from the UA. While efficient, this
constrains the service route for proxy operations to be congruent
with the route taken by the REGISTER message.
6. Returning service route information as the value of a header field
in the REGISTER response. While similar to the previous
alternative, this approach grants the ability for the registrar to
selectively apply knowledge about the topology of the home network
in constructing the service route.
This document defines this final alternative: returning the service
route information as a header field in the REGISTER response. This
new header field indicates a "preloaded route" that the UA may wish
to use if requesting services from the proxy network associated with
the registrar generating the response.
Scenario
UA1----P1-----| |--R-------|
| | |
P2---| DBMS
| | |
UA2-----------| |--HSP-----|
In this scenario, we have a "home network" containing routing proxy
P2, registrar R, home service proxy HSP, and database DBMS used by
both R and HSP. P2 represents the "edge" of the home network from a
SIP perspective, and might be called an "edge proxy". UA1 is an
external UA behind proxy P1. UA1 discovers P1 via Dynamic Host
Configuration Protocol (DHCP) (this is just an example, and other
mechanisms besides DHCP are possible). UA2 is another UA on the
Internet, and does not use a default outbound proxy. We do not show
Domain Name System (DNS) elements in this diagram, but will assume
their reasonable availability in the discussion. The mission is for
UA1 to discover HSP so that outbound requests from UA1 may be routed
(at the discretion of UA1) through HSP, thereby receiving outbound
services from HSP.
Willis & Hoeneisen Standards Track [Page 3]

RFC 3608 SIP Extension for Service Route Discovery October 20033. Discussion of Mechanism
UAs may include a Route header field in an initial request to force
that request to visit and potentially be serviced by one or more
proxies. Using such a route (called a "service route" or "preloaded
route") allows a UA to request services from a specific home proxy or
network of proxies. The open question is, "How may a UA discover
what service route to use?"
This document defines a header field called "Service-Route" which can
contain a route vector that, if used as discussed above, will direct
requests through a specific sequence of proxies. A registrar may use
a Service-Route header field to inform a UA of a service route that,
if used by the UA, will provide services from a proxy or set of
proxies associated with that registrar. The Service-Route header
field may be included by a registrar in the response to a REGISTER
request. Consequently, a registering UA learns of a service route
that may be used to request services from the system it just
registered with.
The routing established by the Service-Route mechanism applies only
to requests originating in the user agent. That is, it applies only
to UA originated requests, and not to requests terminated by that UA.
Simply put, the registrar generates a service route for the
registering UA and returns it in the response to each successful
REGISTER request. This service route has the form of a Route header
field that the registering UA may use to send requests through the
service proxy selected by the registrar. The UA would use this route
by inserting it as a preloaded Route header field in requests
originated by the UA intended for routing through the service proxy.
The mechanism by which the registrar constructs the header field
value is specific to the local implementation and outside the scope
of this document.
4. Applicability Statement
The Service-Route mechanism is applicable when:
1. The UA registers with a registrar.
2. The registrar has knowledge of a service proxy that should be used
by the UA when requesting services from the domain of the
registrar. This knowledge may be a result of dynamic assignment
or some other mechanism outside the scope of this document.
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RFC 3608 SIP Extension for Service Route Discovery October 2003
3. The registrar(s) has/have sufficient knowledge of the network
topology, policy, and situation such that a reasonable service
route can be constructed.
4. The service route constructed by the registrar is the same for all
contacts associated with a single address-of-record. This
mechanism does not provide for contact-specific service routes.
5. Other mechanisms for proposing a service route to the UA are not
available or are inappropriate for use within the specific
environment.
Other methods may also be available by which a UA may be informed of
a service route. Such alternative methods are outside the scope of
this document. Discussion of why one might wish to assign a service
route during registration or when it might be appropriate to do so is
outside the scope of this document.
5. Syntax
The syntax for the Service-Route header field is:
Service-Route = "Service-Route" HCOLON sr-value *( COMMA sr-value)
sr-value = name-addr *( SEMI rr-param )
Note that the Service-Route header field values MUST conform to the
syntax of a Route element as defined in [3]. As suggested therein,
such values MUST include the loose-routing indicator parameter ";lr"
for full compliance with [3].
The allowable usage of header fields is described in Tables 2 and 3
of [3]. The following additions to this table are needed for
Service-Route.
Addition of Service-Route to SIP Table 3:
Header field where proxy ACK BYE CAN INV OPT REG PRA
_______________________________________________________________
Service-Route 2xx ar - - - - - o -
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RFC 3608 SIP Extension for Service Route Discovery October 20036. Usage6.1. Procedures at the UA
The UA performs a registration as usual. The REGISTER response may
contain a Service-Route header field. If so, the UA MAY store the
value of the Service-Route header field in an association with the
address-of-record for which the REGISTER transaction had registered a
contact. If the UA supports multiple addresses-of-record, it may be
able to store multiple service routes, one per address-of-record. If
the UA refreshes the registration, the stored value of the Service-
Route is updated according to the Service-Route header field of the
latest 200 class response. If there is no Service-Route header field
in the response, the UA clears any service route for that address-
of-record previously stored by the UA. If the re-registration
request is refused or if an existing registration expires and the UA
chooses not to re-register, the UA SHOULD discard any stored service
route for that address-of-record.
The UA MAY choose to exercise a service route for future requests
associated with a given address-of-record for which a service route
is known. If so, it uses the content of the Service-Route header
field as a preloaded Route header field in outgoing initial requests
[3]. The UA MUST preserve the order, in case there is more than one
Service-Route header field or header field value.
Loose routes may interact with routing policy in interesting ways.
The specifics of how the service route set integrates with any
locally required default route and local policy are implementation
dependent. For example, some devices will use locally-configured
explicit loose routing to reach a next-hop proxy, and others will use
a default outbound-proxy routing rule. However, for the result to
function, the combination MUST provide valid routing in the local
environment. In general, the service route set is appended to any
locally configured route needed to egress the access proxy chain.
Systems designers must match the service routing policy of their
nodes with the basic SIP routing policy in order to get a workable
system.
6.2. Procedures at the Proxy
The Service-Route header field is generally treated like any other
unknown header field by intermediate proxies. They simply forward it
on towards the destination. Note that, as usual, intermediate
proxies that need to be traversed by future requests within a dialog
may record-route. Proxies should not assume that they will be
traversed by future requests in a dialog simply because they appear
in the Service-Route header field.
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RFC 3608 SIP Extension for Service Route Discovery October 2003
There is a question of whether proxies processing a REGISTER response
may add themselves to the route set in the Service-Route header
field. While this would enable dynamic construction of service
routes, it has two significant problems. The first is one of
transparency, as seen by the registrar: Intermediate proxies could
add themselves without the knowledge or consent of the registrar.
The second problem is interaction with end-to-end security. If the
registrar uses S/MIME techniques to protect the REGISTER response,
such additions would be visible to the UA as "man in the middle"
alterations in the response. Consequently, intermediate proxies
SHOULD NOT alter the value of Service-Route in REGISTER responses,
and if they do, the UA MUST NOT be required to accept the alteration.
Additional considerations apply if a proxy is "dual homed", meaning
connected to two (or more) different networks such that requests are
received on one interface and proxied out through another network
interface. Proxies implementing multi-homing precisely as documented
in [3] record-route a request with the sending interface. When
processing the reply, they replace the Record-Route header field
value that represents the interface onto which they proxied the
request with a new value that represents the interface onto which
they will proxy the response. Consequently, the route vector seen at
the User Agent Server (UAS) is not the exact inverse of the route
vector seen at the User Agent Client (UAC). While in itself
harmless, this complicates matters for nodes that use the recorded
route vector (or recorded Path vector as per [4]) in the
determination of a service route for future use.
Instead of following the procedure in [3], proxies used with
Service-Route that are inserting Record-Route or Path header field
values SHOULD record not one but two route values when processing the
request. The first value recorded indicates the receiving interface,
and the second indicates the sending interface. When processing the
response, no modification of the recorded route is required. This
optimization provides for fully invertible routes that can be
effectively used in construction of service routes.
6.3. Procedures at the Registrar
When a registrar receives a successful REGISTER request, it MAY
choose to return one or more Service-Route header field(s) in the 200
class response. The determination(s) of whether to include these
header fields(s) into the 200 class response and what value(s) to
insert are a matter of local policy and outside the scope of this
document.
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RFC 3608 SIP Extension for Service Route Discovery October 2003
Having inserted a Service-Route header field or fields, the registrar
returns the 200 class response to the UA in accordance with standard
procedures.
A REGISTER operation performing a Fetching Bindings (i.e., no Contact
header field is present in the request) SHOULD return the same value
of Service-Route as returned in the corresponding previous REGISTER
response for the address-of-record in question. In some cases, the
Service-Route may be dynamically calculated by the registrar rather
than stored, and the decision as to whether this route should be
recalculated in the event of a Fetching Bindings operation is left to
the implementation.
Note: A Fetching Bindings operation could be used by the UA to
recover a lost value of Service-Route. Alternatively, a UA in
this situation could just re-REGISTER.
Certain network topologies MAY require a specific proxy (e.g.,
firewall proxy) to be traversed before the home service proxy. Thus,
a registrar with specific knowledge of the network topology MAY
return more than one Service-Route header field or element in the 200
class response; the order is specified as top-down, meaning the
topmost Service-Route entry will be visited first. Such
constructions are implementation specific and outside the scope of
this document.
In general, the Service-Route header field contains references to
elements strictly within the administrative domain of the registrar
and home service proxy. For example, consider a case where a user
leaves the "home" network and roams into a "visited" network. The
registrar cannot be assumed to have knowledge of the topology of the
visited network, so the Service-Route it returns contains elements
only within the home network.
Note that the inserted Service-Route element(s) MUST conform to the
syntax of a Route element as defined in [3]. As suggested therein,
such route elements MUST include the loose-routing indicator
parameter ";lr" for full compliance with [3].
Willis & Hoeneisen Standards Track [Page 8]

RFC 3608 SIP Extension for Service Route Discovery October 20036.4.2. Example of Mechanism in INVITE Transaction
This example shows the message sequence for an INVITE transaction
originating from UA1 eventually arriving at UA2 using outbound
services from HOME.EXAMPLE.COM. UA1 has previously registered with
HOME.EXAMPLE.COM and been informed of a service route through
HSP.HOME.EXAMPLE.COM. The service being provided by HOME.EXAMPLE.COM
is a "logging" service, which provides a record of the call for UA1's
use (perhaps the user of UA1 is an attorney who bills for calls to
customers).
Note that in this example UA1 and UA2 are assumed to be registered
with the same network (HOME.EXAMPLE.COM). This does not generally
need to be the case to use the herein described service route
mechanism.
Message sequence for INVITE using Service-Route:
F1 Invite UA1 -> P1
INVITE sip:UA2@HOME.EXAMPLE.COM SIP/2.0
Via: SIP/2.0/UDP UADDR1.VISITED.EXAMPLE.ORG:5060;branch=z9hG4bKnashds7
To: Customer <sip:UA2@HOME.EXAMPLE.COM>
From: Lawyer <sip:UA1@HOME.EXAMPLE.COM>;tag=456248
Call-ID: 38615183343@s1i1l2j6u
CSeq: 18 INVITE
Contact: <sip:UA1@UADDR1.VISITED.EXAMPLE.ORG>
Route: <sip:P2.HOME.EXAMPLE.COM;lr>,
<sip:HSP.HOME.EXAMPLE.COM;lr>
. . .
Note: P1 is selected using the "outbound proxy" rule in UA1.
F2 Invite P1 -> P2
INVITE sip:UA2@HOME.EXAMPLE.COM SIP/2.0
Via: SIP/2.0/UDP P1.VISITED.EXAMPLE.ORG:5060;branch=z9hG4bK34ghi7ab04
Via: SIP/2.0/UDP UADDR1.VISITED.EXAMPLE.ORG:5060;branch=z9hG4bKnashds7
To: Customer <sip:UA2@HOME.EXAMPLE.COM>
From: Lawyer <sip:UA1@HOME.EXAMPLE.COM>;tag=456248
Call-ID: 38615183343@s1i1l2j6u
CSeq: 18 INVITE
Contact: <sip:UA1@UADDR1.VISITED.EXAMPLE.ORG>
Record-Route: <sip:P1.VISITED.EXAMPLE.ORG;lr>
Route: <sip:P2.HOME.EXAMPLE.COM;lr>,
<sip:HSP.HOME.EXAMPLE.COM;lr>
. . .
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RFC 3608 SIP Extension for Service Route Discovery October 2003
. . .
Note: P2 selected by outbound proxy rule on HSP.
HSP has removed itself from the Route.
INVITE propagates toward UA2 as usual.
7. Security Considerations
It is possible for proxies between the UA and the registrar during
the REGISTER transaction to modify the value of Service-Route
returned by the registrar, or to insert a Service-Route even when one
was not returned by the registrar. The consequence of such an attack
is that future requests made by the UA using the service route might
be diverted to or through a node other than would normally be
visited. It is also possible for proxies on the INVITE path to
execute many different attacks. It is therefore desirable to apply
transitive mutual authentication using sips: or other available
mechanisms in order to prevent such attacks.
The "sips:" URI as defined in [3] defines a mechanism by which a UA
may request transport-level message integrity and mutual
authentication. Since there is no requirement for proxies to modify
messages, S/MIME signed bodies may be used to provide end-to-end
protection for the returned value.
Systems using Service-Route SHOULD provide hop-by-hop message
integrity and mutual authentication. UAs SHOULD request this support
by using a "sips:" URI. Registrars returning a Service-Route MUST
implement end-to-end protection using S/MIME and SHOULD use S/MIME to
protect all such responses. UAs receiving Service-Route SHOULD
authenticate attached S/MIME bodies if present.
Willis & Hoeneisen Standards Track [Page 14]

RFC 3608 SIP Extension for Service Route Discovery October 200311. Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
12. Authors' Addresses
Dean Willis
dynamicsoft Inc.
3100 Independence Parkway
#311-164
Plano, TX 75075
US
Phone: +1 972 473 5455
EMail: dean.willis@softarmor.com
Bernie Hoeneisen
Switch
Limmatquai 138
CH-8001 Zuerich
Switzerland
Phone: +41 1 268 1515
EMail: hoeneisen@switch.ch, b.hoeneisen@ieee.org
URI: http://www.switch.ch/Willis & Hoeneisen Standards Track [Page 16]

RFC 3608 SIP Extension for Service Route Discovery October 200313. Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Willis & Hoeneisen Standards Track [Page 17]